Security support for pdns-recursor ended at 2020-05.
AFAICS it used to be present in stretch, but it was decided to
remove it around 2020-07 for this reason:
"Security support has ended upstream, and it'd be better if
people wouldn't run DNS servers that have known security bugs (and
we are unable to backport those)."
If you want to install it knowing the security implications, you can fetch a past package through http://snapshot.debian.org/ .
Jessie isn't supported anymore by Debian (and pdns-recursor isn't
supported in ELTS), I suppose it was handled differently because
there was no early EOL.
Debian LTS Team
Dear LTS Team,
we are using pdns-recusor as our default dns recursor on all our systems. Starting with Stretch-lts it seems the package disappeared and our preseed installer is not working anymore to install new Debian 9 system (which we still need sometimes).
Is there a reason why this package is now missing? Even Debian jessie still has a candidate in the repository. ☹
Mit freundlichen Grüßen